Multiple Spanning Tree Instances
Multiple spanning tree instances (MSTIs) control the topology inside an MSTP region. An MSTI is a spanning tree domain that operates within and is bounded by a region; an MSTI does not exchange BPDUs with or send notifications to other regions. You must identify an MSTI on a per region basis. The MSTI ID does not have any significance outside of its region so you can reuse IDs across regions. An MSTI consists of a group of VLANs, which can share the same network topology. Each MSTI has its own root bridge and a tree spanning its bridges and LAN segments.
You must first configure a CIST before configuring any MSTIs in the region. You cannot delete or disable a CIST if any of the MSTIs are active in the system.
You can map multiple VLANs to an MSTI; however, multiple MSTIs cannot share the same VLAN.